In my opinion, it's too soon to say how The Avengers will affect the X-Men franchise. Not enough is known about Thor or Captain America. If Thor and Captain America resonate as well with audiences as Iron Man did, then The Avengers could steal some of the X-Men's thunder. Of course, that's assuming that everything falls into place and The Avengers is a hit. Right now, I think the biggest detriment to the X-Men franchise, if any, will be the X-Men films themselves. Audiences will lose interest in the X-Men franchise, if the films continue to decline in quality and fail to meet audience expectations. Other than that, it's too soon to say, as their are too many unknown variables that have yet to come into play.